Structure-activity relationship study towards non-peptidic positron emission tomography (PET) radiotracer for gastrin releasing peptide receptors: Development of [18F] (S)-3-(1H-indol-3-yl)-N-[1-[5-(2-fluoroethoxy)pyridin-2-yl]cyclohexylmethyl]-2-methyl-2-[3-(4-nitrophenyl)ureido]propionamide.

Gastrin-releasing peptide receptors (GRP-Rs, also known as bombesin 2 receptors) are overexpressed in a variety of human cancers, including prostate cancer, and therefore they represent a promising target for in vivo imaging of tumors using positron emission tomography (PET). Structural modifications of the non-peptidic GRP-R antagonist PD-176252 ((S)-1a) led to the identification of the fluorinated analog (S)-3-(1H-indol-3-yl)-N-[1-[5-(2-fluoroethoxy)pyridin-2-yl]cyclohexylmethyl]-2-methyl-2-[3-(4-nitrophenyl)ureido]propionamide ((S)-1m) that showed high affinity and antagonistic properties for GRP-R. This antagonist was stable in rat plasma and towards microsomal oxidative metabolism in vitro. (S)-1m was successfully radiolabeled with fluorine-18 through a conventional radiochemistry procedure. [18F](S)-1m showed high affinity and displaceable interaction for GRP-Rs in PC3 cells in vitro.

Design, synthesis, lipophilic properties, and binding affinities of potential ligands in positron emission tomography (PET) for visualization of brain dopamine D4 receptors.

We report the synthesis of compounds structurally related to the high-affinity dopamine D4 receptor ligand N-{2-[4-(3-cyanopyridin-2-yl)piperazin-1-yl]ethyl}-3-methoxybenzamide (1e). All compounds were specifically designed as potential PET radioligands for brain D4 receptor visualization, having lipophilicity within a range for brain uptake and weak non-specific binding (0.75100-fold), but its D4 receptor affinity was suboptimal for imaging of brain D4 receptors (Ki =30 nM).

Towards metabolically stable 5-HT7 receptor ligands: a study on 1-arylpiperazine derivatives and related isosters.

Serotonin 7 (5-hydroxytryptamine7 or 5-HT7) is the most recently identified serotonin receptor. It is involved in mood disorders and is studied as a target for antidepressants. Here, we report on the structural manipulation of the 5-HT7 receptor ligand 4-[2-(3-methoxyphenyl)ethyl]-1-(2-methoxyphenyl)piperazine (1a) aimed at obtaining 5-HT7 receptor ligands endowed with good in vitro metabolic stability. A set of N-[3-methoxyphenyl)ethyl-substituted] 1-arylpiperazine, 4-arylpiperidine and 1-aryl-4-aminopiperidine was synthesized and tested in radioligand binding assays at human cloned 5-HT7 and 5-HT1A receptors. In vitro metabolic stability of the target compounds was assessed after incubation with rat hepatic S9 microsomal fraction. Among the new compounds, 1-(2-biphenyl)-4-[2-(3-methoxyphenyl)ethyl]piperazine (1d) and 4-(2-biphenyl)-1-[2-(3-methoxyphenyl)ethyl]piperidine (2d) showed a good compromise between affinity at 5-HT7 receptor (K i = 7.5 nM and 13 nM, respectively) and in vitro metabolic stability (26 and 65 % recovery of parent compound, respectively) but were poorly selective over 5-HT1A receptor.

Design, synthesis, and binding affinities of potential positron emission tomography (PET) ligands with optimal lipophilicity for brain imaging of the dopamine D3 receptor. Part II.

In the search for compounds with potential for development as positron emission tomography radioligands for brain D(3) receptor imaging, a series of N-[4-(4-arylpiperazin-1-yl)butyl]arylcarboxamides with appropriate lipophilicity (2

Determination of 1-aryl-4-propylpiperazine pKa values: the substituent on aryl modulates basicity.

In order to design a potential drug, it is important to know its pK(a) because the protonation state of the molecule will be critical for ligand-receptor interaction and for the pharmacokinetic of the molecule. pK(a) values of a series of 1-(substitutedphenyl)-4-propylpiperazines were measured to study how the presence of a substituent on the phenyl ring modulates the basicity of N-4 nitrogen. pK(a) values indicated that the position of the substituent was crucial. In general, the introduction of the substituent in ortho-position of the phenyl ring increased the basicity of the molecule. This effect appeared to be related to steric and conformational effects and not to the electronic properties of the substituent. On the other hand, meta- and para-substituted derivatives showed a slight decrease of pK(a) that was qualitatively consistent with the electronic properties of the substituent.

Design, synthesis, and binding affinities of potential positron emission tomography (PET) ligands for visualization of brain dopamine D3 receptors.

We here report the synthesis of compounds structurally related to the high-affinity dopamine D(3) receptor ligand N-[4-[4-(2,3-dichlorophenyl)piperazin-1-yl]butyl]-7-methoxy-2-benzofurancarboxamide (1). All compounds were specifically designed as potential PET radioligands for brain D(3) receptors visualization, having lipophilicity within a range for high brain uptake and weak nonspecific binding (2 < ClogP < 3.5) and bearing a methoxy substituent for easy access to labeling with the positron emitter isotope (11)C. N-[4-[4-(5-methoxy-2-benzisoxazolyl)piperazin-1-yl]butyl]-4-(4-morpholinyl)benzamide (22), N-[4-[4-(5-methoxy-2-benzisoxazolyl)piperazin-1-yl]butyl]-4-(1H-imidazol-1-yl)benzamide (23), and N-[4-[4-(5-methoxy-2-benzisoxazolyl)piperazin-1-yl]butyl]-5-(2-furanyl)-1H-pyrazole-3-carboxamide (24) displayed good D(3) receptor affinities (K(i) values 38.0, 22.6, and 21.3 nM, respectively) and were selective over D(2) receptor. Moreover, compounds 22-24 were able to permeate the Caco-2 cell monolayer, differently from compound 1. Although the goal to identify potential PET radioligands with subnanomolar affinities for D(3) receptor was not achieved, the proposed strategy could be a starting point for future developments.

Structure-affinity relationship study on N-[4-(4-arylpiperazin-1-yl)butyl]arylcarboxamides as potent and selective dopamine D(3) receptor ligands.

The benzamide PB12 (N-[2-[4-(4-chlorophenyl)piperazin-1-yl]ethyl]-3-methoxybenzamide) (1), already reported as potent and selective dopamine D(4) receptor ligand, has been modified searching for structural features that could lead to D(3) receptor affinity. Changes in the aromatic ring linked to N-1 piperazine ring led to the identification of 2-methoxyphenyl and 2,3-dichlorophenyl derivatives (compounds 6 and 13) displaying moderate D(3) affinity (K(i) = 145 and 31 nM, respectively). Intermediate alkyl chain elongation in compounds 1, 6, and 13 improved binding affinity for the D(3) receptor and decreased the D(4) affinity (compounds 18-26). Among these latter compounds, the N-[4-[4-(2,3-dichlorophenyl)piperazin-1-yl]butyl]-3-methoxybenzamide (19) was further modified with the replacement or of the 2,3-dichlorophenyl moiety (compounds 27-30) or of the 3-methoxyphenyl ring (compounds 31-41). In this way, we identified several high-affinity D(3) ligands (0.13 nM < K(i)'s < 4.97 nM) endowed with high selectivity over D(2), D(4), 5-HT(1A), and alpha(1) receptors. In addition, N-[4-[4-(2,3-dimethylphenyl)piperazin-1-yl]butyl]-3-methoxybenzamide (27) and N-[4-[4-(2,3-dichlorophenyl)piperazin-1-yl]butyl]-7-methoxy-2-benzofurancarboxamide (41) appear to be valuable candidates for positron emission tomography (PET) because of their affinity values, lipophilicity properties, and liability of (11)C labeling in the O-methyl position.

Synthesis, radiolabeling and in vivo evaluation of [(11)C](R)-1-[4-[2-(4-methoxyphenyl)phenyl]piperazin-1-yl]-3-(2-pyrazinyloxy)-2-propanol, a potential PET radioligand for the 5-HT(7) receptor.

In the search for a novel serotonin 7 (5-HT7) receptor PET radioligand we synthesized and evaluated a new series of biphenylpiperazine derivatives in vitro. Among the studied compounds, (R)-1-[4-[2-(4-methoxyphenyl)phenyl]piperazin-1-yl]-3-(2-pyrazinyloxy)-2-propanol ((R)-16), showed the best combination of affinity, selectivity, and lipophilicity, and was thus chosen for carbon-11 labelling and evaluation in pigs. After intravenous injection, [(11)C](R)-16 entered the pig brain and displayed reversible tracer kinetics. Pretreatment with the 5-HT7 receptor selective antagonist SB-269970 (1) resulted in limited decrease in the binding of [(11)C](R)-16, suggesting that this radioligand is not optimal for imaging the brain 5-HT7 receptor in vivo but it may serve as a lead compound for the design of novel 5-HT7 receptor PET radioligands.

3-(1H-indol-3-yl)-2-[3-(4-nitrophenyl)ureido]propanamide enantiomers with human formyl-peptide receptor agonist activity: molecular modeling of chiral recognition by FPR2.

N-formyl peptide receptors (FPRs) are G protein-coupled receptors (GPCRs) that play critical roles in inflammatory reactions, and FPR-specific interactions can possibly be used to facilitate the resolution of pathological inflammatory reactions. Recent studies indicated that FPRs have stereo-selective preference for chiral ligands. Here, we investigated the structure-activity relationship of 24 chiral ureidopropanamides, including previously reported compounds PD168368/PD176252 and their close analogs, and used molecular modeling to define chiral recognition by FPR2. Unlike previously reported 6-methyl-2,4-disubstituted pyridazin-3(2H)-ones, whose R-forms preferentially activated FPR1/FPR2, we found that four S-enantiomers in the seven ureidopropanamide pairs tested preferentially activated intracellular Ca(2+) flux in FPR2-transfected cells, while the R-counterpart was more active in two enantiomer pairs. Thus, active enantiomers of FPR2 agonists can be in either R- or S-configurations, depending on the molecular scaffold and specific substituents at the chiral center. Using molecular modeling approaches, including field point methodology, homology modeling, and docking studies, we propose a model that can explain stereoselective activity of chiral FPR2 agonists. Importantly, our docking studies of FPR2 chiral agonists correlated well with the FPR2 pharmacophore model derived previously. We conclude that the ability of FPR2 to discriminate between the enantiomers is the consequence of the arrangement of the three asymmetric hydrophobic subpockets at the main orthosteric FPR2 binding site with specific orientation of charged regions in the subpockets.

The therapeutic potential of 5-HT1A receptors: a patent review.

INTRODUCTION: The 5-HT(1A) receptors are implicated in mood disorders (anxiety, depression), in cognition, and in modulation of pain. Nearly 30 years of research in this field, there is still interest in developing new chemical entities capable of 5-HT(1A) receptor activation or blockade. AREAS COVERED: This review article will highlight and review the research advances published in the patent literature between January 2007 and December 2011, giving emphasis to the medicinal chemist's standpoint. Literature search methodology included search in Scifinder and PubMed Databases and browsing clinicaltrials.gov website. EXPERT OPINION: Almost no new therapeutic applications have been proposed for molecules targeting the 5-HT(1A) receptor, during the years covered by the present review. The discovery that stimulation of 5-HT(1A) receptor can promote neurogenesis will likely renew the interest for selective 5-HT(1A) receptor agonists as therapeutics to replace neural populations damaged by disease or injury.

Investigations on the 1-(2-biphenyl)piperazine motif: identification of new potent and selective ligands for the serotonin(7) (5-HT(7)) receptor with agonist or antagonist action in vitro or ex vivo.

Here we report the design, synthesis, and 5-HT(7) receptor affinity of a set of 1-(3-biphenyl)- and 1-(2-biphenyl)piperazines. The effect on 5-HT(7) affinity of various substituents on the second (distal) phenyl ring was analyzed. Several compounds showed 5-HT(7) affinities in the nanomolar range and >100-fold selectivity over 5-HT(1A) and adrenergic α(1) receptors. 1-[2-(4-Methoxyphenyl)phenyl]piperazine (9a) showed 5-HT(7) agonist properties in a guinea pig ileum assay but blocked 5-HT-mediated cAMP accumulation in 5-HT(7)-expressing HeLa cells.

Design, synthesis, radiolabeling, and in vivo evaluation of carbon-11 labeled N-[2-[4-(3-cyanopyridin-2-yl)piperazin-1-yl]ethyl]-3-methoxybenzamide, a potential positron emission tomography tracer for the dopamine D(4) receptors.

Here we describe the design, synthesis, and evaluation of physicochemical and pharmacological properties of D(4) dopamine receptor ligands related to N-[2-[4-(4-chlorophenyl)piperazin-1-yl]ethyl]-3-methoxybenzamide (2). Structural features were incorporated to increase affinity for the target receptor, to improve selectivity over D(2) and σ(1) receptors, to enable labeling with carbon-11 or fluorine-18, and to adjust lipophilicity within the range considered optimal for brain penetration and low nonspecific binding. Compounds 7 and 13 showed the overall best characteristics: nanomolar affinity for the D(4) receptor, >100-fold selectivity over D(2) and D(3) dopamine receptors, 5-HT(1A), 5-HT(2A), and 5-HT(2C) serotonin receptors and σ(1) receptors, and log P = 2.37-2.55. Following intraperitoneal administration in mice, both compounds rapidly entered the central nervous system. The methoxy of N-[2-[4-(3-cyanopyridin-2-yl)piperazin-1-yl]ethyl]-3-methoxybenzamide (7) was radiolabeled with carbon-11 and subjected to PET analysis in non-human primate. [(11)C]7 time-dependently accumulated to saturation in the posterior eye in the region of the retina, a tissue containing a high density of D(4) receptors.

Structural modifications of N-(1,2,3,4-tetrahydronaphthalen-1-yl)-4-aryl-1-piperazinehexanamides: influence on lipophilicity and 5-HT7 receptor activity. Part III.

Starting from the previously reported 5-HT 7 receptor agents 4-7 with N-(1,2,3,4-tetrahydronaphthalen-1-yl)-4-aryl-1-piperazinehexanamide structure, the 1-(2-methylthiophenyl)-, 1-(2-diphenyl)-, 1-(2-isopropylphenyl)-, and 1-(2-methoxyphenyl)piperazine derivatives 8-31 were designed with the primary aim to obtain new compounds endowed with suitable physicochemical properties for rapid and extensive penetration into the brain. The affinities for 5-HT 7, 5-HT 1A, and D 2 receptors of compounds 8-31 were assessed, and several compounds displayed 5-HT 7 receptor affinities in the nanomolar range. Among these, N-(4-cyanophenylmethyl)-4-(2-diphenyl)-1-piperazinehexanamide (25) showed high 5-HT 7 receptor affinity (Ki = 0.58 nM), high selectivity over 5-HT 1A and D 2 receptors (324- and 245-fold, respectively), and agonist properties (maximal effect = 82%, EC 50 = 0.60 microM). After intraperitoneal injection in mice, 25 rapidly reached the systemic circulation and entered the brain. Its brain concentration-time profile paralleled that in plasma, indicating that 25 rapidly and freely distributes across the blood-brain barrier. Compound 25 underwent N-dealkylation to the corresponding 1-arylpiperazine metabolite.